The benzo(a)pyrene (BP) metabolite pattern formed by monocytes and lymphocytes includes three dihydrodiols, a major 7,8-diol, low or moderate levels of 4,5-diol and barely detectable levels of 9,10-diol, at least three phenols, the 3-OH, 7-OH and 9-OH and two major separable quinones, the 1,6 and 3,6. BP metabolite patterns formed by benz(a)anthracene (BA)-induced monocytes and lymphocytes from the same individual show that each cell type has characteristic differences in the relative ratios of the BP metabolites formed. Relatively greater proportions of 7,8-diol and 9-OH are synthesized by monocytes, while lymphocytes form relatively higher levels of 3-OH and the BP-quinones. Thus, it appears that monocytes and lymphocytes contain different patterns of cytochrome P-450 mixed function oxidase or metabolically related enzymes which are active in BP metabolism. The ratio of BP metabolites formed by BA-induced to that of control monocytes and lymphocytes are greater for the 7-OH and 9-OH and lesser for the 3-OH and 7,8-diol in most samples examined. The total amount of BP metabolites formed are quantitatively higher in BA-induced cells. This data suggests that induction by BA in these cells is selective and favors the formation of certain forms of cytochrome P-450 which change the metabolite patterns in induced cells.